This invention relates to the field of monovinylaromatic compound purification and polymerization, and more particularly discloses a process for the reduction of phenylacetylene contaminants in crude styrene feedstock prior to polymerization of the styrene into polystyrene.
Of all the thermoplastics manufactured today, probably the most versatile and most widely utilized class of materials is polymerized monovinyl aromatic compounds such as polystyrene, polymerized alpha-methyl styrene, and polymers of ring-substituted styrenes.
Some of the most common uses of these compounds (often referred to collectively as xe2x80x9cstyrenesxe2x80x9d or xe2x80x9cpolystyrenesxe2x80x9d) are for manufacturing food and beverage containers, food wrap, and children""s toys. One disadvantage associated with such uses of polystyrene is the residual monomer and other contaminants in the polymer, which may contribute to off-taste, odor, off-color and other adulteration or degradation of the polymer quality.
A particularly offensive contaminant associated with such undesirable properties in polystyrene is unreacted vinyl aromatic monomer, usually styrene monomer. One of the causes of unreacted monomer is directly related to the presence of phenylacetylene in the styrene feedstock going into the polymerization reactor system.
In the manufacture of monovinyl aromatic polymer compounds, and more particularly in the manufacture of polystyrene (PS), benzene is reacted with ethylene to form ethylbenzene (EB). This molecular compound is then dehydrogenated in an EB dehydrogenation, or xe2x80x9cdehydroxe2x80x9d, unit to form a crude styrene product. The crude styrene product is subsequently purified to produce styrene monomer product. The styrene monomer is then polymerized, usually in the presence of a polymerization initiator or catalyst, to form the final polystyrene raw material.
Unfortunately, phenylacetylene, one of the undesirable side products of the EB dehydro unit, is formed when EB is dehydrogenated one step too far. Consequently, the product stream from the dehydro unit contains styrene, EB, and traces of phenylacetylene. The EB is easily removed by conventional processes, such as common distillation, leaving styrene monomer and phenylacetylene. The removal of phenylacetylene cannot be accomplished by simple or conventional means such as distillation and has heretofore been a difficult and very costly process.
The presence of phenylacetylene in styrene monomer has undesirable consequences regardless of whether the method of polymerization utilized is anionic, or free-radical polymerization. During anionic polymerization, phenylacetylene which is slightly acidic, consumes a stoichiometric amount of catalyst, such as butyllithium, wherein one molecule of butyllithium is removed from the polymerization process by each molecule of phenylacetylene. This loss of catalyst can be costly and causes the concentration of catalyst to be difficult to control. This, in turn, causes the molecular weight of the polystyrene to be difficult to control and can result in an increase in the concentration of low molecular weight polymer and even leave unreacted styrene in the polystyrene.
During free-radical polymerization, the presence of phenylacetylene can have detrimental effects on chain length and polymerization rate, because it is a poor chain transfer agent. Consequently, in the manufacture of polystyrene beads, which are used to make expanded polystyrene (EPS) or xe2x80x9cfoamedxe2x80x9d polystyrene, significant amounts of residual styrene are left in the beads. Styrene creates undesirable taste, color, and odor, even when present in only minute amounts in the polymer.
Thus, the presence of phenylacetylene in styrene monomer has adverse effects on cost, control of the polymerization process, and purity of the resulting polystyrene. The presence of phenylacetylene in polystyrene also results in olefinic bonds in the backbone of the polymer which can increase cross-linking and cause more rapid oxidation of the polymer, both of which degrade the polymer significantly.
In the free-radical polymerization of styrene, as the concentration of styrene decreases during the polymerization process, the relative concentration of phenylacetylene naturally rises. Since phenylacetylene acts as a polymerization inhibitor, the polymerization process is undesirably affected.
Catalytic attempts at reducing the phenylacetylene levels in styrene monomer streams have involved the injection of high levels of hydrogen gas into the monomer in an attempt to reduce the phenylacetylene to styrene. Any hydrogen added into the stream in stoichiometric excess of the phenylacetylene present there results in conversion of significant amounts of styrene back into ethylbenzene, causing a lower styrene concentration and a lower conversion rate. Significant reductions in phenylacetylene were achieved only at the expense of styrene conversion to EB and resultant loss of styrene production.
One patent directed to the use of hydrogen gas for phenylacetylene reduction (PAR) is U.S. Pat. No. 5,156,816 granted to Butler et al on Oct. 20, 1992, which teaches a PAR process based upon the use of a catalytic bed with multiple hydrogen injection; dilution of the hydrogen by diluents such as nitrogen, carbon dioxide and carbon monoxide; using EB ventgas to supply a hydrogen and diluent combination; and, using a multiple catalyst bed reactor, or multiple reactors to achieve hydrogenation. In this patent, the written description and drawings of which are hereby incorporated herein by reference in their entirety, a preferred catalyst for the dehydrogenation reaction was palladium on an alumina carrier.
One problem with the above-incorporated PAR process is that the Pd/A1 catalyst used in the PAR reactor to dehydrogenate phenylacetylene will continually lose palladium from the alumina carrier until the conversion rate of PA to styrene becomes unacceptably low and the catalyst has to be removed and replaced with new catalyst. Attempts to use various additives to increase conversion of PA to styrene and to increase selectivity of the catalyst from converting styrene to converting PA, have met only minimal success and have not solved the problem of palladium stripping.
The present invention solves the problems of the prior art by providing an additive when added to conventional PAR systems that not only increases the level of PA conversion, but also stabilizes the catalyst and prevents stripping of the palladium from the alumina base. The additive is one which would normally be utilized as a styrene polymerization inhibitor, in the class of inhibitors consisting of hydroxylamines, as well as combinations of hydroxylamines with phenylene diamines and oxime compounds.